Discussion on the problems needing attention in th

2022-08-24
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Discussion on the problems that should be paid attention to in the tensile strength test of coating paint

discussion on the problems that should be paid attention to in the tensile strength test of coating paint

December 15, 2018

0 preface cellulose nitrate prepared by the esterification of a mixture of nitric acid and sulfuric acid with cellulose, and a nitrocellulose coating paint coated on aircraft masking cloth made with mixed solvents such as esters, alcohols, aromatic hydrocarbons, etc., has the advantages of fast drying, high viscosity, light color, good transparency, tensile strength Good flexibility and extensibility. Painting on the cloth has good shrinkage, which makes the cloth airtight and impervious, and can improve the tensile strength and shrinkage of the cloth. Tensile strength refers to the strength of the film's resistance to tensile force, that is, the load applied to the film per unit cross-sectional area (the free film on the cloth before the test) when the free film breaks or plasticizes after the tensile test; The free coating refers to the Chinese coating with a certain thickness when the coating paint is applied, which will change with the height of the cloth surface before it is completely dried, and has a certain fluidity

1 test part 1.1 measuring instruments and material equipment

electronic balance: sensitivity 0.01 ~ 0.1 g, Beijing analytical instrument factory; Wool brush: 50 ~ 65 mm wide; Tensile testing machine: measuring range: 0 ~ 200kg, Shanghai textile machinery company; Wood frame: 560 mm × 400 mm (calculated according to the inner side), the width of the wooden strip on each side is 25 mm, and the thickness is 35 mm; Mengbu: No. 114 silk light flat cloth, Shijiazhuang aircraft manufacturing company; Metal stylus; Slotted screwdriver; Glass plate: 500 mm × 400 mm; Ruler: 20 cm, division value is 0.5 mm; 500 ml stainless steel cup

1.2 determination reagents

anhydrous ethanol (gb/t678), butanol (gb/t12590), ethyl acetate (gb/t12589), butyl acetate (hg/t3498), all of which are analytical pure, Tianjin North Tianyi Chemical reagent; Toluene (gb/t684) and acetone (gb/t686) are all analytical pure, Tianjin kemio chemical reagent. The nitro mixed diluent prepared by the above reagents in different proportions, and the preparation ratio of the mixed solvent is: n (toluene): n (ethanol): n (butanol): n (ethyl acetate): n (butyl acetate) = 50:7:10:11:22

1.3 measurement steps

first, according to the length and width of the wood frame, cut a piece of masking cloth that is 3 ~ 5 cm larger than the outer edge of the length and width of the wood frame, and lay it flat on the wood frame. At both sides of the outer edge of the wood frame, the radial reduction is 4 mm, and the upper and lower latitudinal reduction is 3 mm. Take out a piece of fabric wire respectively, a total of 4 pieces of fabric wire, slightly force the thread drawing seam to align with the outer edge of the wood frame, press a thumbtack every 4 cm, and after tightening the upper frame of the masking cloth, the masking cloth should be moderately loose, The center of the cloth cannot collapse. Weigh the cloth on the balance after putting it on the frame, and the weight of the cloth and the frame before painting is W1 (g). Because the viscosity of the coating paint is too high, the viscosity value measured by Tu-1 viscometer is usually more than 150 S. in order to make the free film painted on the cloth evenly distributed, it is necessary to add the fault analysis and troubleshooting method of the material testing machine, add an appropriate amount of nitro diluent to dilute it, and then paint it, but the nitro diluent dose shall not exceed 20% of the original paint amount, The usage of coating paint is still calculated according to the original paint. Brush the coating paint under the condition of constant temperature and humidity [(25 ± 2) ℃, 50% ± 5%]; Use a paint brush to brush two layers alternately in the warp and weft directions of the cloth. The paint brush should not be moved many times. The amount of paint used for each coating (calculated by wet film) is 160 ~ 180 g/m2, and the amount of paint applied for each layer should be less than the actual weight gain of 18.75 g/m2 (calculated by dry film). The interval time of each layer is: the actual drying time after the first, second, third and fourth coats of coating paint are 20 ~ 30 min, 30 ~ 45 min, 45 ~ 60 min and 45 ~ 60 min respectively. After painting, after the film is dried for 24 hours under constant temperature and humidity, the weight of the coated cloth and frame is W2 (g), and the weight gain of the coated cloth (g/m2) is calculated according to the formula 1.4.1, that is, the sum of the paint amount of each layer should be less than the actual weight gain of 75 g/m2 (calculated by dry film) after four coats of paint are painted

after the film is dried for 72 hours under the condition of constant temperature and humidity, remove the masking cloth from the frame, place it flat on the glass plate, divide the strips with a metal stylus, measure the cloth sample with a ruler on the uncoated surface of the masking cloth with a length of 280 mm and a width of 52 mm according to the meridional and latitudinal directions required by the tensile test, and then use a metal stylus to separate the coated cloth strips along the meridional and latitudinal directions of the masking cloth with a measured size, and accurately measure the width of 50 mm, Take out the redundant lines on both sides from beginning to end, and cut off the rough edges with scissors. On the tensile testing machine, the distance between the upper and lower clamps is adjusted to 20 cm, and the lower collet lowering speed is 100 ~ 110 mm/min. After calibrating the center of the test strip, stretch it. When the test strip breaks, record the pointer reading immediately. According to the same method, the tensile test of unpainted warp and weft masking strips shall be carried out respectively

1.4 calculation method

1.4.1 calculation method of weight gain of masking cloth after painting

weight gain of masking cloth after painting x (g/m2) is calculated according to formula (1)

where: W1 - weight of masking cloth and frame before painting, G

w2 - weight of the cloth and frame after painting, G

s - painting area, cm2

1.4.2 calculation method of tensile strength increase of masking cloth after painting

(1) tensile strength increase of meridional masking cloth δ Calculation of warp (kg/m) according to formula (2):

in the formula: F warp - tension of warp facing cloth strip before painting, kg

f '- warp tensile force of test strip after painting, kg

b - width of test strip (equal to 50 mm), mm

f 'is the arithmetic mean value of the tension of three meridional test strips with similar values, and the difference between the measured value of any one of them and the mean value is not greater than 3% of the mean value

(2) increase in tensile strength of zonal masking cloth δ The weft (kg/m) is calculated according to formula (3):

where: F weft - weft direction masking strip tension before painting, kg

f '- weft tensile force of weft test strip after painting, kg

b - width of test strip (equal to 50 mm), mm

f 'weft is the arithmetic mean of the pull of three weft test strips with similar values, and the difference between the measured value of any one of them and the mean value is not greater than 3% of the mean value

(3) the increase of tensile strength after the masking is painted δ (kg/m) calculated according to formula (4):

2 screening and analysis of test data

carry out the tensile test on a tensile testing machine with a measurement range of 0 ~ 200 kg. Test 5 groups of blank specimens in parallel, take 3 groups of 6 data with similar meridional and latitudinal values, and calculate the average value respectively. The difference between the measured value of any one of them and the average value is not greater than 3% of the average value, as shown in Table 1

warp and weft blank test data

carry out tensile test on the test piece coated with masking cloth on this tensile testing machine. When the test strip deforms to fracture, write down the pointer reading, take three groups of 6 data with similar values in the warp and weft directions, calculate the increase in tensile strength (kg/m) of the warp and weft Mongolian cloth respectively, and then calculate the total tensile strength (kg/m) of the Mongolian cloth. The difference between the measured value of any one of them and the average value is not more than 3% of the average value, as shown in Table 2

the data of tensile strength increase of warp and weft montmorillonite

after calculation, the arithmetic mean of tensile strength increase of warp and weft montmorillonite is 952.7 and 922.7 respectively, and the arithmetic mean of total tensile strength of montmorillonite is 1875.3; The difference between each result and the average value is no more than 3% of the average value, which fully meets the requirement that the total tensile strength

of the product standard masking cloth is greater than 1850 kg/cm

3 problems and preventive measures to reduce the amount of graphene added to 1/510 of the earliest samples available. According to the feedback of users over the years and the visits to large enterprises across the country, it is found that the following problems often occur in the detection process, and the corresponding preventive measures are put forward for the problems

(1) after brushing, the free film has partial condensation and uneven distribution on the coated surface of the cloth before leveling. The main reasons for this are: because the coating paint is a volatile paint, the surface drying time is very short, and the coating film has dried before leveling. The viscosity (Tu-1 viscometer) specified in the coating paint product standard is 120 ~ 180 S. due to the characteristics of the coating paint itself, the coating will be diluted during storage, In order to prevent the viscosity of polystyrene foam from being widely used in the manufacture of disposable coffee cups and a variety of building materials from being reduced to the lower limit or below during the storage period, the viscosity of the coating paint is usually greater than 160 s when it is offline. Therefore, an appropriate amount of dilution should be carried out according to the viscosity of the coating paint during the detection process, but the amount of nitro dilution should not exceed 20% of the amount of the original paint

(2) whitening, spots, stripes. The main reason for this is that the construction site is wet, the atmosphere contains a lot of water, the solvent volatilizes too fast, the surrounding temperature drops sharply, and the water vapor condenses on the film, so that a small part of nitrocellulose precipitates to form whitening, spots, and stripes on the film; Preventive measures: add an appropriate amount of organic solvent with high boiling point and slow volatilization speed, and the temperature at the construction site should not be lower than 12 ℃; At the same time, pay attention to the humidity of the construction site and the volatilization rate of the solvent used. Generally, the relative humidity of the construction environment should not be greater than 70%, otherwise there will be whitening, which will reduce the tensile strength of the cloth

(3) there are drops and bubbles on the back of the mask. The main reasons for this are: too much thinner (volatile liquid) is added to the paint, resulting in too low viscosity, and the force is too large when brushing, so that the paint sample with too low viscosity penetrates to the back through the surface of the cloth, forming drops; In addition, the sample has not fully wetted the cloth, and the brushing speed is too fast, forming a large number of bubbles on the surface of the cloth; Preventive measures: strictly control the dosage of diluent according to the viscosity of the product. After adding the diluent, fully stir it evenly, and do not make a small part of the diluent float on the coated paint sample

(4) twill appears on the test masking strip. The main reasons for this are: the drawing line is not neat, or there are interrupted lines. Due to the existence of interrupted lines, no gap can be found, resulting in twill on the warp and weft lines when stretching the wooden frame, resulting in twill on the coated cloth

(5) when making tensile strength tensile test strips, sometimes a word fracture occurs suddenly at the initial stage of tension. The main reasons for this are: because the coating thickness of the coated test piece is uneven, the film-forming content of the coating is usually low, usually about 10%, so the number of brushing should be at least 4 times evenly; In addition, the blank test of the cloth is vacant, which causes the cloth that cannot meet the standard to be applied to the test, so the blank test of the cloth must be done batch by batch, and the cloth that has not done the blank test is not allowed to be used

4 conclusion the tensile strength test results have very strict requirements for the amount of coating paint used, and the tensile strength measurement results of warp and weft Mongolian cloth will be affected by various aspects and produce errors. The error of tensile strength test comes from the error of test results caused by uneven application of coating paint on the cloth, the error of the size of the cloth, the error of viscosity between coating paint batches, the error of the tightness of the cloth when putting on the wooden frame, the accuracy of the pressure testing machine, the temperature and humidity of the test environment and the error caused by personnel operation. The size of the coated surface of the cloth specified in the standard is 560 mm × 400 mm, while the size of the cloth strip participating in the test is only 200 mm × 50 mm, the influence of these errors on the measured results of such a small cloth strip should not be underestimated. In addition, the brushing method of the masking cloth also determines that the free film on the masking cloth is not uniform everywhere. The uneven test piece is difficult to represent the quality level of the tensile strength of the sample. This requires that by measuring the amount of dry film used in each layer, the stable fence type aluminum based lead alloy composite anode plate products that do not meet the test interval should be discarded, so that the impact of comprehensive errors is relatively reduced and the inspection accuracy is relatively improved

the above is the author's test on the tensile strength of coating paint for many years

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